The National Cancer institute, Division of Cancer prevention and control, Chemoprevention Branch has initiated an investigational agent development program for chemopreventive agents with potential for inhibiting the process of carcinogenesis. The purpose is to obtain new agents suitable for clinical trials in humans who do not have cancer but may be at increased risk for cancer. Agents being developed by the chemoprevention Branch must be tested for toxicity in animals; all toxicologic data and a summary of toxicity observed in animals treated with the agent must be submitted to the Food and Drug Administration as part of the investigational New Drug Application for human clinical trials. The objective of this project - carcinogenicity Study of Difluoromethyl ornithine (DFMO) in Rats - is to evaluate the potential of DFMO to cause cancer in F344 rats when administered for 104 consecutive weeks.